The storage of supercritical CO2 in reservoirs capped by impervious rock formations
(Carbon Capture and Storage, Carbon Sequestration or, simply, CCS) has recently
been proposed as an approach to reducing atmospheric emissions of greenhouse
gases. Nevertheless, these techniques could lead to the potential damage of the
hosting rock as a result of gas overpressure. Thus, forecasting its behaviour is of
utmost importance to avoid fluid migration, a situation that would reduce storage
efficiency, pollute nearby aquifers and even trigger seismic events due to fault
reactivation.
This thesis is aimed at studying several aspects concerning Carbon Sequestration,
paying special attention to how the development of such injection projects could affect
geological media. The first part of the thesis approaches the topic from a more
theoretical perspective, intoducing the concept of CCS and reviewing the most
relevant geotechnical theory that rule multiphase flow and fracture behaviour. The
work is then focused on the importance of numerical modelling and, in particular, the
use of the finite element code CODE_BRIGHT to simulate this type of problems. In a
more practical approach, an interesting real case, the In Salah injection project, is
selected as an example of how the existence of highly conductive, fractured features
can alter the expected behaviour of geological media. In a simplified manner, and in
an attemp to understand the particular aspects of this case, some 2-D simulations
have been carried out.
Finally, a typical reservoir-caprock environment as found in CO2 storage schemes is
simulated using data provided by several papers (Rutqvist, 2008, 2010 & 2013).
[ANGLÈS] Simulations are carried out assuming three different scenarios for a similar geometry
under damaged and undamaged conditions. It is concluded that the existence of
fractures (although inititally closed) in the sealing caprock could trigger an
uncontrollable migration of supercritical CO2 to upper strata and could also lead to
much bigger, and also uncontrolled, vertical displacements (uplifts). The particular
case in which a vertical fault (modeled as a damaged area rather than a single
discontinuity plane) exists is studied. In this case, the zone where the vertical feature
intersects the reservoir is especially critical. It is there where the maximum decrease
in effective net stresses is observed and, thus, it is a potential failure point that could
also propagate fault inestability by allowing CO2 to keep moving upwards if the
increment of permeability is big enough. The formation of a clearly defined
preferential path is observed, and CO2 is allowed to keep spreading through the fault
until it reaches a steady state far from the injection reservoir.